Iron powder and the manufacture of magnetic cores therefrom



Feb. 14, 1961 'r A. DUNTON arm. 2,971,872

mom POWDER AND THE MANUFACTURE OF MAGNETIC CORES THEREFROM Filed Nov. 27, 1957 THORNLEYHTHELSTHNDUNTON DHVID KENNETH WORN Inventors B GLO.

A Horn 2 y United States Patent IRON POWDER AND THE MANUFACTURE OF MAGNETIC CORES THEREFROM Thornley Athelstan Dunton and David Kenneth Worn, Birmingham, England, assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware 7 Filed Nov. 27, 1957, Ser. No. 699,285 Claims priority, application Great Britain Sept. 16, 1954 2 Claims. (Cl. 148-105) The present invention relates to magnetic cores used at very high frequencies as well as to a special iron powder used in making these magnetic cores, and more particularly to a carbonyl iron powder, the fine particles of which have aggregated during decomposition of the carbonyl to form an open mesh and hereinafter termed fibrous powders, of improved electromagnetic properties for use in high frequency electromagnetic cores.

It is well known that the manufacture of metal powders from their carbonyls was originally developed by Mond in 1890. The use of this powder for electrical purposes by the Germans during World War II has been described in British Intelligence Objectives Sub-Committee (B1. 0.8.), Final Report No. 1575 (Interrogation of Dr. Leo Schlecht, pages 20-28). Carbonyl iron powder is presently used to make magnetic cores for induction coils for use at very high frequencies. It is often important for very high frequency use that the coils be high Q coils and that the core of the coil increases the Q or merit of the coil. The Q of a core coil assembly is defined as the ratio of the inductive reactance to the elfective re sistance and can be expressed as follows:

where L is the inductance in henries, w is ZFXfrequency in cycles per second and R is the total effective resistance. R comprises not only the resistance of the winding, R,,, but also the efiective resistance caused by eddycurrent, hysteresis and other losses occurring in the, core when the latter is in an alternating field. The winding resistance at the test frequency may be written as f is the frequency in cycles per second L is the efiective inductance in henries U is the core permeability B,,, is the maximum flux density in gauss a is the hysteresis loss constant c is the residual loss constant e is the eddy current loss constant To this must be added the A.C. resistance of the coil. It will be noted that for low frequency applications the principal core losses are the residual and hysteresis losses. At very high frequencies, eddy current losses are prelimit the quality monly called carbonyl iron powder. We have also ex- 7 plained that the size of the powder particles is important, and as the frequency at which the core is used increases it becomes more and more desirable to use as fine a powder as possible.

In the usual method of manufacture, the powder, before being compressed into cores, is mixed with insulating material, which forms insulating layers between the powder particles, and generally also with an additional binder to increase the mechanical strength of the core'. The insulating material may be, for instance, sodium silicate, and the binder a phenol-formaldehyde resin. It is well known that the magnetic properties of the cores are greatly influenced by the degree and character of insulation between the particles. When the powders are very fine, having a large proportion of particles of a size well below 3 microns (as determined by a Fisher sub- ,sieve sizer) difiiculties are encountered in insulating them. -In-particular, if the same percentage of insulation is used -a sjwiith larger particle powders, the film of insulation, being thinner, tends to break down during pressing, caus- Qing particle-to-particle contact.

According to the above-mentioned B.I.O.S. Final Report No. 1575, page 21, the grade E and the grade EN powders described therein are the powders used for high frequency application.' The letter N after the *E" signified that the powder had been ball milled. Grade E powder particles are mostly spherical in shape. A photograph of this grade E powder appears in Goetzel, Treatise on Powder Metallurgy, volume II, 1950, ed., Interscience Pub. Inc., N.Y., page 293. It is exceedingly difiicult to produce grade E spherical powder below 3 microns. The usual procedure used for obtaining small particle size powder consists in separating a fine fraction from a spherical powder having 'a higher average particle size. When attempting to make powders below 3 microns directly in the decomposer, it is often found that although the particle size as determined by a Fisher sub-sieve sizer is indeed below this value, the individual particles are often aggregates of particles of even finer particle size, the overall dimensions of the open mesh being greater than 3 microns. Because of this fibrous nature, it is extremely diflicult to properly insulate these particles and part of the invention in our previous patent application Ser. No. 534,398, filed September 14, 1955, as shown particularly in Example I thereof, consists essentially in forming an iron sulfide film on the no surface of the fibrous particles, this film improving the electrical insulation between the irregular shaped particles. This treatment improved the Q values of cores considerably, but naturally the further elimination of unwanted eddy currents which are a great disadvantage is desirable.

It has now been discovered that a powder consisting of fibrous aggregates of primary particles may be so milled or further milled after the formation of theiro'n sulfide film as to break up at least a substantialpropor 0 tion of the clusters or aggregates without breakingup the primary particles themselves to any substantial We find that the Q values of the resultant cores made from v 3 these fibrous powders are improved, particularly at high frequencies.

It is an object of the present invention to provide a fibrous carbonyl iron powder with exceedingly high Q values when formed into a core for use with an induction coil.

Other objects and advantages will become apparent from the following description taken in conjunction with the drawings in which:

Figure 1 is a reproduction at 3500 magnification showing fibrous powder particles.

Figure 2 is likewise a reproduction of a photomicrograph taken at 3500 magnification showing the structure of unmilled fibrous powder particles.

Figure 3 is a reproduction of a photomicrograph taken at 3500 magnification showing the structure of milled fibrous powder particles produced in accordance with the invention.

Generally speaking, the present invention contemplates the production of iron cores for induction coils useful in very high frequency application by producing fibrous powder in a carbonyl decomposer, insulating the powder with hydrogen sulfide, ball milling the powder, mixing the powder with sodium silicate and a binder and making a core.

of a photomicrograph taken the structure of unmilled charge of steel balls weighing over 5 times as much as the powder charge.

The Q values obtained from cores made of the unmilled and milled powder respectively were as follows:

. 'Q values at frequencies (in Pressure, me./s.) 05- tons/sq. in.

Umnilledpowder; g z y Mmedpmer --l 2% 2% 33:11: 3% ii ii Other batches of the powder were sulfided in conical flasks by passing hydrogen sulfide gas over them for 2, 3.75 and 4 minutes approximately giving sulfur contents of 0.33%, 0.99% and 1.12%. Partof'each batch of sulfided powder was milled in the same way as the starting powder. In addition, part of the batch of milled starting powder was sulfided to a sulfur content of 0.74%, and some of this milled and sulfided powder was then again milledfor 16 hours.

The Q values obtained from cores made from the powder after these various treatments were as follows:

Aiter'SuIfidlng After sulfidlng and Mining i Pres/surg; lgei'lcrent ons sq. ur

20 50 100 200 20 50 100 200 McJs. McJs- McJs. Me./s. McJs. Mc.ls. Mc./s. Mc./s.

'20 0 187 218 113 84 192 247 140 103 {45 155 121 52 38 190 216 113 80 20 n} 0 178 224' 120 '89 185 247 150 115 45-.- I62 160 72 55v 185 222. 125 98 -20.. 112 177 185 93 69 188 248 154 121 45..- 99 65 29 26 185' 218 119 92 20-.- o 74 186 243 138 99 189 253 150 113 45--- 183 231 123 89 186 232 129. 98

into practice, it has been found that good results may also be obtained by sulfiding the powder, then ball milling the powder, then insulating the powder with sodium silicate, adding an acetone solution of phenol-formaldehyde resin as a binder and compressing the resultant powder mixture into a core, which may take a convenient form suchas a cylinder.

For the purpose of giving those skilled in the art a better appreciation of the advantages of the invention, illustrative examples are given below.

The improvements produced are shown by comparative tests in which batches of a single starting fibrous In carrying the invention powder were difierently treated and formedinto cores,

the Q values of the cores at various frequencies then being ascertained.

The starting fibrous powder had a mean particle size of 2.45 microns, and was produced directly by decomposition of carbonyl in a decomposer and contained 2.57% carbon, 1.03% nitrogen-and 0.001% sulfur. The method employed in making each core consisted of stirring the powder in an insulating material such as an aqueous solution of sodium silicate at about 60 C. until the mixture was dry, adding an acetone solution of 'a henolformaldehyde resin and again stirring until the acetone had evaporated; form'm'Jg the powder, which'contained 1% sodium silicate and 3% resin, i'n't'o cylindrical-cores under pressure; and finally heating at 130 C. for 2 hours to polymerise the resin. Two cores were made from the starting powder and two from each treated powder,

one core in each pair being pressed at 20 tons per square inch-and the other at 45 tons per square inch.

The starting powder had very poor insulating characteristics and cores made from it had very low Q values even :at low frequencies. One batch of the starting powderwas milled in a-2 inch diameter and 2 inch long ball forg16hours in a mill containing inch diameter steel balls and rotating at 70 revolutions per minute, the

produced fibrous From the two tables given above it will be seen that milling alone produced a very marked improvement in the Q values at all frequencies, but that at all the higher frequencies this improvement in itself was not nearly as great as that produced by sulfidin'g, and'milling'the powder.

The invention is particularly applicable to directly powders having very fine primary particlesto be made into cores for use at high frequency.

It is to be observed that the present invention provides producing fibrous carbonyl iron powder with a mean particle size of 2.45 microns directly from a carbonyl decomposer, sulfidingthe powder, ball milling the powder, mixing the powder with sodium silicate as a further insulating materialand aphenol-formaldehyde resin binder and compressing the powder into a core having ahigh Q and useful at very high frequencies.

Furthermore, the invention provides the ball-milling to be done either after the sulfiding treatment or before and after the-sulfidingtreatment.

Moreover, the inventionprovides sulfiding the powder by passing hydrogen sulfide gas over the powder for from 2 to 4 minutes approximately-giving a sulfur content of from 0.33% to 1.12%.

The present application is a continuation-in-part of our copending application Serial No. 534,398, filed September 14, 1955.

Although the jarcsentinvention has been described in conjunction with preferred embodiments, itis to be understood that modifications and variations maybe resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered'to be within the purview and scope of the invention and appended claims.

We claim:

induction coils wherein the cores are pressed from iron powder mixed with a aqueous solution of sodium silicate and a phenol-formaldehyde resin binder, the improvement which comprises sulfiding a fibrous carbonyl iron powder having a mean particle size below about 3 microns with hydrogen sulfide gas to a sulfur content of from 0.3% to 1.2% by weight of iron powder for a period of time of from about 2 minutes to about 4 minutes, and subjecting the powder to a ball milling treatment with a charge of steel balls weighing over 5 times the weight of the powder, whereby higher Q values at very high frequencies are obtained in cores made therefrom.

2. In a method for making electromagnetic cores pressed from iron powder, the improvement which comprises ball milling a fibrous carbonyl iron powder having a mean particle size below about 3 microns with a charge of steel balls weighing over 5 times the weight of the powder, sulfiding the ball milled powder with hydrogen sulfide gas to a sulfur content of from 0.3% to 1.2% by weight of iron powder for. a period of time of from about 2 minutes to about 4 minutes and thereafter again subjecting the powder to a ball milling treatment with a charge of steel balls weighing over 5 times the weight of the powder, whereby higher Q values at very high frequencies are obtained in cores made from the thustreated powder mixed with sodium silicate and a phenolformaldehyde resin binder.

References Cited in the file of this patent UNITED STATES PATENTS 1,669,648 Bandur May 15, 1928 2,399,019 Grinter et a1. Apr. 23, 1946 2,666,724 Beller Jan. 19, 1954 2,791,561 Beller et a1. May 7, 1957 

1. IN A METHOD FOR MAKING ELECTROMAGNETIC CORES FOR INDUCTION COILS WHEREIN THE CORES ARE PRESSED FROM IRON POWDER MIXED WITH A AQUEOUS SOLUTION OF SODIUM SILICATE AND A PHENOL-FORMALDEHYDE RESIN BINDER, THE IMPROVEMENT WHICH COMPRISES SULFIDING A FIBROUS CARBONYL IRON POWDER HAVING A MEAN PARTICLE SIZE BELOW ABOUT 3 MICRONS WITH HYDROGEN SULFIDE GAS TO A SULFUR CONTENT OF FROM 0.3% TO 1.2% BY WEIGHT OF IRON POWDER FOR A PERIOD OF TIME OF FROM ABOUT 2 MINUTES TO ABOUT 4 MINUTES, AND SUBJECTING THE POWDER TO A BALL MILLING TREATMENT WITH A CHARGE OF STEEL BALLS WEIGHING OVER 5 TIMES THE WEIGHT OF THE POWDER, WHEREBY HIGHER Q VALUES AT VERY HIGH FREQUENCIES ARE OBTAINED IN CORES MADE THEREFROM. 